Download openwrt ipk packages
openwrt_url = 'https://downloads.openwrt.org/chaos_calmer/15.05/'
ar9331_packages = 'ar71xx/generic/packages/'update-ipk.py: get Packages and then check local repos, download if not exist or MD5Sum mismatch.
GET: https://downloads.openwrt.org/chaos_calmer/15.05/ar71xx/generic/packages/base/Packages.gz
status=200, content-type="application/octet-stream", content-length=121975
GET: https://downloads.openwrt.org/chaos_calmer/15.05/ar71xx/generic/packages/luci/Packages.gz
status=200, content-type="application/octet-stream", content-length=90639
......
check-ipk.py: check local repos, generate rm-orphanfiles.cmd to delete duplicated files.